What is particularly damaging in terms of the potential working life of optical media (light, dirt etc.)? Does the colour of the disc also play a role and, if so, why?

Light, heat, atmospheric humidity, scratches and dirt all play a role in determining wear.The colour of the recording layer plays no role. It is merely an indication of the chemical substance used in the recording layer. Invisible to the customer, the differences lie in the quality of this substance itself, which can vary greatly.